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52 The Sclera

Cross one another at various angles, and are often united by bands which pass between the adjacent bundles ; these fibre arcuate are especially conspicuous in the anterior lamelle. The peculiarity of the substantia propria in yielding after boiling a modified form of chondrin, instead of' the usual gel tin, has already been mentioned.

The cellular elements the corneal corpuscles, are flattened, plate like connective tissue cells which lie between the lamelle of the fibrous stroma within the intercommunicating lymph spaces hollowed out within the cement substance. The corneal cells are irregularly branched, and form, by means of their united processes, a protoplasmic network throughout the corneal stroma The corneal spaces in which the cells lie are larger than the cells, and are therefore only partially filled by the protoplasmic elements, the unoccupied space affording channels for the circulation of the nutrient tissue juices upon which the investment of the non vascular cornea depends. Communication between the corneal spaces is established by the canaliculi which pass from one space to the other. The corneal cells usually are applied to one wall of the spaces, and, in principle, resemble the endothelial plates which line other and larger lymphatic cavities. Occasional migratory leukocytes, or wandering cells, are also found within the system of corneal juice channels.

The posterior limiting membrane, membrane of Descemet, membrane of Demours, or posterior elastic membrane, appears as a sharply defined homogeneous band from 0.010 to 0.012 Dim. in its thickest peripheral portion, at the inner boundary of the substantia propria. It differs from the anterior limiting membrane in its marked resistance to acids, alkalies, boiling water, and other reagents; it resembles, but is by no means identical with, elastic tissue. It is capable of complete separation from the substantia propria after prolonged maceration in a 10 per Cent.solution of sodium chlorid. The layer in question contains no cells, and ordinarily presents no indication of being composed of secondary lamelle, although sometimes after reagents it shows traces of such structure.

The relations of the posterior limiting membrane at the corneal periphery are of interest, since in this position it breaks tip into numerous bands which are continued into the trabecule forming the pectinate ligament of the iris.

The posterior endothelium covers the, inner surface of the membrane of Descemet and forms part of the lining of the anterior chamber of the eve. This innermost stratum of the cornea is composed of a single layer of' polybedral plates, the Outlines Of which constitute a mosaic of considerable regularity. The cells closely resemble ordinary endothelial plates, possessing oval, sometimes reniform, nuclei which are usually of greater thickness than the surrounding cell body. The endothelium and the membrane of Descemet are of importance as constituting almost impassable barriers to the escape of the aqueous humor into the lymph channels of the cornea.

The blood vessels of the normal fulIy developed cornea are limited to an extremely Darrow peripheral zone, about I mm. in width, the remaining portions of the cornea being entirely devoid of blood channels. The vascular zone contains the terminal loops of the episcleral branches derived from the anterior ciliary arteries. The venous radicles become tributaries of the anterior ciliary veins.

The nerves of the cornea constitute a rich supply arranged in the form of numerous plexuses. The corneal nerves are derived from the ciliary plexus, contributed by the long and short ciliary nerves, and form an annular plexus in the vicinity of the corneal margin. The twigs from the annular plexus pass either directly or indirectly to the corneal tissue, those destined for the anterior layers first having joined the conjunctival nerves before proceeding to the cornea. The more numerous branches which pass directly to the corneal stroma from the annular plexus enter the substantia propria near the Posterior limiting membrane, the far greater number, however, passing to the anterior lamella, only about one third of the nerves which enter the cornea being distributed to the posterior lavers. The nerve bundles, on penetrating into the corneal stroma are invested for a short distance, from 0 75 1 mm by perineural lymph sheaths, the individual nerve fibers losing their medullarv sheaths at about the same time.

After entering the substantia propria the nerves form the fundamental plexus within the corneal stroma, from which numerous lateral branches are given off at various levels ; these are composed of non medullated fibers which soon break up into the component varicose fibrille In addition to the lateral twigs, performating branches ascend through the, anterior lamelle as far as the epithelium, beneath which they form the subepithelial plexus. The terminal fibers of this plexus in many instances enter the epithelium to end either in special end bulbs or between the cells as the intra epithelial plexus. The plexuses within the substantia propria formed by the twigs given off at various levels spread out between the lamelle of fibrous tissue; the nodal points or places of meeting of the fibers are often marked by angular areas outlined by the interlacing fibers; nuclei, belonging to the 'delicate nerve are sometimes present. The terminal fibers of the corneal nerves are related to various forms of end organs, among which are intricate convolutions, less contorted loops and hooks, and irregular quadrate plates.

The Sclera. The sclerotic coat forms the posterior four fifths of the fibrous tunic of the eyeball, contributing largely to the protection and support of the more delicate structures within, as well as affording the points of attachment of the ocular muscles. Although composed of practically the histological elements as the cornea, the disposition of these is such that the dead white opacity is produced which so conspicuously contrasts with the beautifully transparent cornea.

The sclera is thickest over the posterior third of the ball where the maintenance of a uniform curvature for the support of the retina is of great importance in the vicinity of the optic nerve the sclerotic coat measures nearly I mm. in thickness, gradually becoming thinner toward the anterior boundary, until beneath, or just posterior to, the zone of attachment of the recti muscles the sclera is reduced to about 0.4 mm. Anterior to the tendonzone the thickness of the fibrous tunic is augmented by the expansion of the muscle insertions until it reaches about 0.6 mm. In individuals possessing thin sclere and deeply pigmented eyes the sclerotic coat presents a bluish or skimmed milk tint, due to the deeply colored tissue beneath the fibrous coat; this bluish appearance is well marked in the eyes of young children.

In its structure the sclera closely resembles the cornea, being composed of interlacing bundles of fibrous tissue disposed with much greater irregularity, however, than those of the cornea. The clefts between the fibrous bundles correspond to the corneal spaces and contain irregularly stellate connective tissue cells the scleral corpuscles the scleral spaces are less regularly arranged and possess a less elaborate system of connecting canaliculi. The scleral bundles further differ from those of the cornea in containing numerous elastic fibers and in yielding gelatin on boiling: their general disposition is equatorial and meridional, although the bundles interlace with one another at all angles.

In addition to the usual branched scleral corpuscles, those occupying the innermost stratum are deeply pigmented, in consequence of which the inner surface of' the sclerotic coat presents a dark color and is known as the lamina fusca: this layer constitutes the outer wall of the subscleral IN" mph space, and is attached to the subjacent choroid by numerous trabecule, which, together with the limiting walls of the space, are covered with endothelial plates. The greater extent of the outer surface of the sclera, from the sheath of the optic nerve to the insertion of the ocular muscles, is also clothed with endothelium, which forms part of the lining of the episcleral space of Tenon.

The blood vessels of the sclera, in addition to the perforating vessels, which include anterior branches from the anterior ciliary vessels, the large equatorially situated vene vorticose, and posterior branches from the posterior ciliary vessels, are represented by the meager twigs within the superficial strata of the fibrous tunic derived from the wide meshed episcleral network formed by branches derived from the anterior and posterior ciliary arteries. The sclera receives additional branches from the short ciliary arteries in the vicinity of the optic entrance : these small vessels are of interest, since from the circulus Zinnii, which they form within the fibrous coat around the optic nerve, minute twigs extend into the dural nerve sheath and anastomose with the arterioles supplying the sheath derived from the central artery of the retina, thus establishing a communication between the retinal and choroidal circulation.

The veins which drain the scleral coat are tributary to three sets of vessels: those from the anterior tract, emptying into the anterior ciliary veins; those from the equatorial zone, joining the vene vorticose ; and those from the posterior part, pouring their blood into the posterior ciliary veins.

The lymphatics of the sclera are represented by the system of intercom scleral spaces, those in the vicinity of the sclero corneal Juncture being in close relation with the spaces of Fontana at the angle of the anterior chamber, which they indirectly aid in draining.

The nerves distributed to the sclerotic coat consist of a few twigs derived from the ciliary nerves as these pass between the sclera and choroid, which terminate between the fibrous bundles of the superficial layers as tortuous and intricately coursing ultimate fibrille.

The relations of the scleral tissue to the sheaths surrounding the optic nerve will be considered with the description of the Optic Entrance.

The Sclero corneal Juncture. The position at which the sclera and corneal segments of the fibrous coat meet is one of the most important regions of the eye, since in the immediate vicinity of this junction lie important channels through which escapes the aqueous humor as well as the fibers giving origin to the ciliary muscle.

The conspicuous Iine of union between cornea and sclera depends far more upon the physical differences of the two portions of the fibrous coat than upon actual structural variation, since the elements are not only almost identical, but directly continuous. When seen in section the scleral tissue extends along both margins farther forward than does the corneal substance, the effect of this arrangement being to receive the cornea with an apparent annular groove bounded by the outer and inner scleral processes: of these the inner is shorter and does not reach as far toward the anterior pole as the outer.

The connections of the inner scleral process are of especial importance on account of the relations to the structures marking the meeting of the cornea, the iris, and the ciliary muscle. Just anterior and external to the process a distinct, usually somewhat irregularly elliptical, open the position of the annular venous sinus, the canal of Schlemm This channel, also called the circulus venosus ciliaris, as seen in sections, elliptical or pyriform in its transverse figure, measures about 0.3 and 0.045 mm. in the longest and shortest diameters respectively. The walls of the canal of Schlemm differ greatly in character, the outer boundary being dense, while the inner is composed of a spongy reticulated.

Layer apparently the continuation of the inner scleral process. The inner wall closely united with the posterior limiting membrane of the cornea anteriorly, and internally with the pectinate ligament of the iris and meridional fibres of the ciliary muscle. The character of Schlemm's canal, whether a venous or lymphatic channel, was a long a subject of active controversy: the recent investigations of leber, however, have brought the formerly opposed views into harmony by showing that the conflicting evidence, based upon carefully conducted observations, was due to conditions of intraocular tension under which the experiments were carried out. It may be regarded as definitely established that the canal of the spaces of Fontana stands in close relation to the anterior chamber on the one hand, and directly communicates with the anterior ciliary veins on the other. under usual conditions schlemm's canal contains but little blood a fact which is explained by Schwalbe upon the supposition that the sinus is an annular reserve diverticulum for the reception and storage of blood when for reason there is a temporary retardation to the escape of the blood passing thorough the anterior ciliary veins; the narrowness of the communicating branches between Schlemm's canal and the scleral veins under ordinary conditions favoring the more direct passage of the contents of the scleral veins into the anterior ciliary vessels, rather than its entrance into the canal.

The tissue forming the wall of the anterior chamber at its angle, occupying the space between it and the canal of Schlemm is peculiar in character, being composed of an aggregation of interlacing trabecule composing a spongy mass containing interfascicular clefts, the spaces of Fontana. These spaces constitute a system of intercommunicating lymph channels which are imperfectly lined with endothelial plates and freely communicate with the anterior chamber, the aqueous humor filling the spaces.

The spongy tissue containing the spaces of Fontana collectively constitutes an annular prismoidal mass, the apex of which begins at the corneal margin, where the membrane of Descemet splits up into delicate bands: these bands mark the origin of the trabecule which pass toward the iris and constitute the ligamentum pectinatum iridis, a rudimentary structure in man representing the much more conspicuous series of conical processes extending from the iris toward the cornea in ruminants. The imperfect character of the endothelial lining of the spaces of Fontana allows the ready entrance of the lymph contained within the anterior chamber, so that the clefts between the trabecule are filled with the escaped aqueous humor ; the loose nature of the septum forming the inner wall of Sehlemm's canal is also favorable to the passage of fluids, in consequence of which arrangement the aqueous humor is continually passing, under normal conditions of intraocular tension, through the Spaces of Fontana into the canal of Schlemm, and thence into the communi venous radicles. This exit for the intraocular lymph is of the utmost importance in maintaining equilibrium of tension within the eyeball.

The Vascular Tunic. The middle or choroidal coat of the eyeball, distinguished by its dark color, and therefore often called the uveal tract is essentially a sheet of vascular connective tissue. It includes three distinct portions the choroid, the ciliary region, and the iris and extends from the optic nerve to the pupil. The character of its component structures renders the nutritive coat soft., friable, and extensible, and, owing to the presence of muscular tissue within its ciliary and iridial segments, it is subjected to constant variations in its tension. The blood vessels of this tunic constitute the chief nutritive apparatus of the eye, since the functionally most active portions of the organ, as the percipient layers of the retina and the ciliary muscle, receive their nutrition from this source.

The choroid constitutes the posterior two thirds of the vascular tunic, extending from the optic nerve entrance to the anterior limit of the visual portion of the retina, or ora serrata, lying closely united to the functionating of the nervous tunic, to the nutrition of which it ministers. The thickness of the choroid gradually diminishes toward the ora serrata, being about 0.1 mm. near the nerve and 0.06 mm. at the ora serrata. While applied to the inner surface, of the sclera the union between the two coats is not firm, since the opposed surfaces, covered with endothelium, are separated by the intervening suprachoroidal lymph space; irregular trabecule extend across this space, and, in addition to attaching the sclera and choroid imperfectly, subdivide the Cleft into numerous secondary compartments. When separated from the fibrous coat the outer surface of the choroid appears rough and ragged, owing to the adherent torn trabeculue. The suprachoroidal space is also occupied by the large vascular and nervous trunks which traverse the cleft in their course to other parts of the eyeball; those which pierce the Sclera, as the vene vorticose aid in further uniting the vascular and fibrous tunics. The inner surface of the choroid, on the contrary, is very intimately united with the adjacent pigmented laver of the retina, so that the latter often adheres to the choroid when the middle coat is removed. The choroid consists of a more or less compact connective tissue stroma, which supports numerous blood channels of very varying size; the arrangement of these vessels largely determines the peculiarities of the layers into which the choroid is divided (Fig. 30). These are three:

  1. The layer of choroidal stroma containing blood vessels of large size
  2. The layer of dense capillary networks the chorio capillaris
  3. The homogeneous glassy lamina or membrana vitrea.
The loose layer of trabecular bands connecting the outer surface of the choroid and the inner surface of the sclera constitutes the lamina Suprachoroidea, sometimes described as an additional layer of the choroid. The membrane like trabecule consist of interlacing fibro elastic bundles, upon the surface of which lie the flattened, irregularly branched pigmented connective tissue cells, the deeply pigmented protoplasm rendering them conspictious elements.

The choroidal stroma consists of a ground substance of closely interwoven connective tissue lamelle,which support the blood vessels. The structural elements include the usual bundles of white fibrous tissue, numerous elastic fibers, and stellate pigmented cells; the stroma is especially dense in the immediate vicinity of' the blood channels.

The layer containing the large blood vessels constitutes the larger part of the choroid, the vascular canals appearing as apertures and lighter channels within the darker choroidal stroma. The largest vessels occupy the most superficial or outer stratum of the choroidal stroma, those of medium size the middle layer, while the innermost layer is devoted to the capillary network, the chorio capillaris.

The most conspicuous of the large superficial blood channels are the four venous trunks, the vene vorticose ; these pierce the choroid within the equatorial zone at points about equidistant and establish foci toward which the smaller veins within each quadrant converge; these tributaries form peculiar venous whorls within the superficial layers of the choroidal stroma (Fig. 31). The vene vorticose traverses the suprachoroidal space, invested by a partial envelope contributed by the lamina suprachoridea, and pierce the sclera, running obliquely backward. Perivascular lyinpli sheaths usually invest the venous trunks within the choroids. The arteries with the choroidal stroma possess longitudinally disposed muscle bundles in addition to the customarv circular fibers.

A narrow boundary zone separates the laver containing the large veins from the capillary stratum: it consists of closely felted fibro elastic fibers intermingled with sparingly distributed connective tissue cells devoid of pigment. In many animals, as the horse, cow, or sheep, the boundary zone contains manv bundles of dense connective tissue, which arrangement produces the peculiar metallic reflex sometimes seen in such eyes; this shining laver constitutes the tapetum, fibrosum, as distinguished from the tapetaum cellulosum of the carnivora, which structure depends upon the presence of 'everal lavers of cells containing minute crystals.

The inner capillary zone of the choroid, the chorio capillaris or membrane of Ruysch, occupies the inner portion of the vascular tunic lying next the vitreous membrane, which alone separates the rich vascular layer from the nervous coat, to the nutrition of which it so largely ministers. The capillaries are unusually uniform in size, measuring about 0.009 rum. in diameter; the meshes of the network are very small, even surpassing in closeness those of the lungs, being only 0.01 to 0.62 mm in the macular region, and about 0,02 to 0.03 mm toward the ora serrata. The red reflex seen in the eye when viewed with the ophthalmoscope is due to the reddish color of this vascular layer showing through the retina.

The vitreous membrane, lamina basilaris, membrane of Bruch, or lamina vitrea, constitutes the inner boundary of the choroid, lying next the nervous tunic, which it separates from the chorio capillaris. The membrane represents a specialized condensation of the choroidal stroma, and appears as a homogeneous zone which measures only 0.002 mm. in thickness.

The nerves of the choroid are derived from branches given off from the long and short ciliary nerves during their course between the vascular and fibrous tunics. The choroidal nerves, which are both medullated and nonmedullated, form a wide mesbed plexus within the lamina suprachoroidea containing groups of ganglion cells. From this plexus numerous slender, non medullated fibers proceed to the arteries, the muscular tissue of which they especially supply ; isolated or very limited groups of ganglion cells are found along the blood vessels.

The lymphatics of the choroid are probably represented by distinct capillary vessels which communicate with the lymph spaces between the channels of the chorio capillaris on the one hand, and the perivascular sheaths tributary to the larger lymph canals on the other.

The ciliary body includes the middle segment of the vascular tunic, extending from the ora serrata behind to the sclero corneal juncture in front. As seen in meridional sections, this region appears as a triangle, the longer and outer side of which lies next the sclera, and selero corneal juncture, the short anterior side against the pectinate ligament, and the inner margin in apposition with the irregular, deeply pigmented extension of the retinal tunic.

The ciliary body presents three subdivision s the ciliary ring, the ciliary processes, and the ciliary muscle.

The ciliary ring, or orbiculus ciliaris, include, the smooth annular tract lying between the sinuous border of the ora serrata behind and the ciliary processes in front, constituting a band about 4 mm. in width. This zone differs in its structure from the choroid proper, chiefly in the absence of the rich vascular supply, since the capillary layer ceases at the ora serrata, or at the point where the percipient elements of the nervous tunic end for whose nutrition the chorio capillaris is especially designed. The larger blood vessels of the choroid are here represented by the venous trunks which return the blood from the iris and ciliary processes and proceed as tributaries to the Vene vorticose. When viewed from tile posterior surface the ciliary ring presents numerous delicate radial striations: these are due partly to the blood vessels and partly to minute plications of the surface, best marked toward the anterior boundary of the ring.

The ciliary processes appear oil the posterior surface of the ciliary region as all annular series of pyramidal folds, about seventy in number, the conspictious projecting bases of which encircle the attached border of the iris, while their apices gradually fade away in the orbiculus ciliaris. The delicate radial striations seen on the surface of the latter are so related to the ciliary processes that each projection seemingly begins by the fusion of several striations, and rapidly increases in breadth and height to a point opposite the margin of the crystalline lens, and then abruptly diminishes to the level of the iris. The elevations measure between 2 and 3 mm. in length, 0. 12 to 0.15 mm in breadth, and in their boldest part from 0.8 to I mm. in height. The processes consist chiefly of convoluted blood vessels supported by delicate connective tissue stroma, and covered by the pigmented extension of the retinal tunic, the pars ciliaris retine It is probable that the particular function of the ciliary processes, in addition to affording attachment for the fibers of the suspensory ligament of the lens, is the secretion of the aqueous humor, to which end their peculiar formation and unusual vascularity are especially adapted.

When seen in meridional sections each process is observed to be composed of a number of irregular projections, varying greatly in size and arrangement (Fig. 29) ; in general, the maximum elevation marks the inner angle next the iris, from which point they gradually diminish toward the orbicular ring, where they fade away. In addition to the connective tissue stroma containing the rich convolution of blood vessels, the inner surface of the ciliary processes, as well as that of the orbiculus ciliaris, is covered by a continuation of the vitreous membrane Of the choroid, which in this region is somewhat thickened, measuring from 0.003 to 0.004 mm.; this limiting membrane separates the stroma of the ciliary process from the retinal laver represented by tile double stratum of epithelial cells which covers the inner surface of the projections.

The ciliary muscle is very conspicuous in meridional sections of the eyeball, then appearing as a triangular fold of involuntary muscle and connective tissue which lies between the sclera and the proper tissue of the ciliary processes. In its entirety the ciliary muscle forms a prismoidal annular band which surrounds the angle of the anterior chamber and attached margin of the iris. The muscular area consists of three sets of bundles of involuntary muscle, intermingled with connective tissue, arranged as meridional, radial, and circular fibers. The meridional bundles are closely grouped and constitute a compact muscular layer next the sclera, to which they are loosely connected by fibers of the lamina suprachoroidea. These muscular bundles take origin from the inner scleral process and tissue, forming the inner wall of Schlemm's canal; posteriorly, the meridional bundles are attached to the choroidal tract, into which they are inserted by delicate tapering processes; from their relation to the vascular tunic the meridional muscular bundles are often called the tensor choroidea. The typical meridional fibers lie next the sclera; those more internally situated gradually assume a more radial disposition, and insensibly blend with those whose course is such that they constitute the radial group (see Fig. 29).

The radial fibers of the ciliary muscle are less closely placed than the meridional, and form a reticulum in which the muscular bundles are separated by a considerable amount of intervening connective tissue. The fan shaped mass of radial fibers diverges from their point of origin from the membrane of Descemet and inner wall of Schlemm's canal, the innermost fibers passing toward the ciliary processes and the outer to the anterior border of the orbiculus ciliaris.

In addition to the meridional and radial bundles an isolated group of circularly disposed muscular fibers occupies the inner angle of the triangular field formed by the ciliary muscle at the base of the iris; these fibers constitute the circular or ring muscle of Muller.

The general form of the ciliary muscle in the emmetropic eye approximates a right angled triangle, the hypothenuse corresponding to the long scleral margin : in the markedly abnormal refractive conditions of myopia and hypermetropia the circular fibers are respectively atrophic or over developed, which results in the obtusely angled myopic muscle and the unusually acute angled muscle of the hypermetropic eye.

The blood vessels of the ciliary body are derived from the anterior and long ciliary arteries, which form around the root of the iris the anastomotic ring, the circulus iridis major. In their course through the ciliary muscle to gain the periphery of the iris these vessels give off twigs which pass directly to the muscle substance ; the arteries supplying the ciliary processes pass backward from the circulus iridis major piercing the inner part of the muscle to reach the anterior extremities of the elevations. The veins returning the blood from the ciliary muscle pass principally into the anterior ciliary trunks : additional venous radicles, however, convey a part of the blood in the opposite direction to join that returned from the ciliary processes by the posteriorly coursing vessels, which finally become tributary to the great equatorial veins.

The nerves of the ciliarv body include sensory, motor, and sympathetic fibers derived from the anterior branches of the long and short, ciliary tru nks these nerves form an annular plexus, the orbiculus qangliosus, within the ciliary muscle. Four sets of fibers probably exist within the ciliary body : 1, sensory fibers, largely subscleral in distribution ; 2, vaso motor fibers distributed to the walls of the blood vessels ; 3, motor fibers ending within the muscular tissue of the ciliary body ; 4, fibers terminating within the interfascicular tissue of the ciliary muscle.

The iris constitutes the anterior segment of the vascular tunic, and is visible, on looking through the clear cornea, as the delicate ' contractile, variously tinted septum which contains the central aperture or pupil. The plane of the iris is not quite vertical, as its pupillary margin rests upon the anterior surface of the lens, which causes slight convexity of its plane. The thickness of the curtain is about 0.04 mm. in the quiescent condition, in a widely dilated state being nearly doubled. The diameter of the iris is about 11 mm., of which the pupil appropriates from 3 6 mm. when at rest (see also p. 147).

The attached or ciliary border of the iris joins the ciliary body behind, and is continuous with the membrane of Descemet through the pectinate ligament in front ; its zone of attachment lies about 3 mm. behind the apparent corneal margin as viewed from before., The exact outline of the thin pupillary border is difficult to see, owing to its intense black color due to the deeply pigmented tissue which forms the immediate boundary of the opening : critically examined, it presents a slightly irregular or dentated contour.

The color of the iris, as viewed from the anterior surface, varies greatly, and depends for its production upon two factors the deeply pigmented cells covering the posterior surface of the iris as well as lining the pupillary opening, and the amount of pigment contained within the iridial stroma. When the pigmented stroma cells are very few or absent the (lark color of the posterior layer shines through the thin stroma, and the iris appears blue ; when the stroma is thicker the tint becomes modified to gray. With the presence of additional pigment within the stroma varying deeper shades, as green, hazel, brown, are produced ; finally, when the stroma is laden with pigmented cells, the darkest tints of' brown appear the so called " black eyes" (see also page 147).

The color is not uniform, but is distributed in irregular spots and patches, sometimes of fanciful form, of lighter and darker tints, so that a definite tint is produced only on viewing the iris at a distance sufficient to blend the variously tinged areas. Close examination shows a further disposition of the color in two zones concentric with the pupil the pupillary, from 1 2 mm wide, which is lighter in dark eyes and darker in light eyes, and an outer or ciliary, from 3 4, mm. in width, which is darker in dark eyes and lighter in light eyes. The boundary zone between the two is often marked by a series of festoon like ridges, the circulus minor iridis.

The anterior surface of the iris, when viewed from before, exhibits a distinct sculpturing consisting in numerous radial striate ridges ; these are particularly fine and closely approximated within the pupillary zone, where they unite toward the inner margin, leaving deep intervening clefts. The broader ciliary portion is subdivided into three secondary zones concentric with the pupil an inner smooth ring, not plicated (luring dilatation of' the pupil ; a middle furrowed band; and in outer irregularly pitted marqinal or cribriform zone. The first two are visible in the living eye, the third is covered by the scleral border.

The posterior surface of the iris presents numerous radially arranged ridges separated by intervening furrows, which are intersected by concentric lines; within the pupillary zone the concentric markings almost disappear, while the radial are more numerous than elsewhere, resulting in the apparent, plication of the inner zone of the iris.

The form of the human pupil is normally circular under all conditions of' contraction ; in marked contrast are the elliptical or slit like pupils of many mammals, in some of which, as the horse and ox, the long axis of the contracted pupil is horizontal ; in others, as the cat and tiger, vertical.

The structure of the iris, as seen in radial sections, presents two chief layers the iridial stroma proper and the pigment layer; these include five sub layers (Fig. 32):

  1. Anterior endothelium
  2. Anterior boundary layer;
  3. Vascular stroma layer;
  4. Posterior limiting layer
  5. Pigment laver.
Reference to the development of the iris shows that the pigment layer is the contribution of the nervous tunic, and morphologically represents the anterior edge of the secondary optic vesicle, derived from the ectoderm, while the remaining parts of the iris are mesodermic in origin.

The anterior endothelium forms part of the lining of the anterior chamber, and consists of a single layer of irregular polygonal plates, directly continuous with those covering the posterior surface of the cornea.

The anterior limiting membrane does not exist as a distinct layer, being simply the modified and condensed subendothelial stratum of the general stroma into which it blends. The connective tissue cells are here unusually closely placed, with a corresponding meagerness of the intercellular fibrous tissue; minute interfascicular clefts represent a system of intercom municating lymph spaces. Blood vessels are wanting within this part of the iris.

The vascular stroma layer, forming the bulk of the iris, consists of a loose connective tissue supporting the numerous blood vessels and nerves which occupy this stratum, and enclosing interfascicular lymph spaces, as well as the groups of involuntary muscle bundles which constitute the sphincter and dilatator papille muscles. The radially disposed blood vessels and nervetrunks are invested by local condensations of the iridial stroma, the perivascular sheaths so formed representing the most robust portions of the stroma layer, the intervening areas being occupied by a comparatively loose connective tissue reticulum.

The variable and often large amount of pigment contained within the stroma layer in dark irides occurs as irregular accumulations of pigment cells, the anterior layer and the pupillary zone usually containing the greatest number of the colored elements. In very dark irides the distribution of the pigment is very general, all portions of the stroma layer being filled with the colored particles.

The muscular tissue within the iris occurs within the vascular stroma layer, and includes the well marked circular fibers surrounding the inner margin of the iris and constituting the sphincter pupille, and the much less evident and often disputed radially disposed fibers which form the dilatator pupille.

The sphincter pupille consists of an annular band of involuntary muscle, varying in width between 0.7 and 1.0 mm., according to the condition of contraction, and from 0.07 to 0.10 min. in thickness. The immediate edge of the pupil is not formed by the muscular tissue, since the pigmented retinal sheet intervenes. The muscle occupies the posterior plane of the stroma layer, behind the blood vessels; the bundles composing its outer border are loosely disposed, certain fibers often assuming an arched course and fading away in radial offshoots.

While the presence of a sphincter muscle is universally admitted, the existence of a radially disposed dilatator pupille is by no means undisputed. The demonstration of a distinct layer of radiating fibers is very unsatisfactory, so much so that many competent observers have concluded that such fibers do not exist, and that a true dilatator is absent, although the presence of radially disposed delicate spindle cells is indisputable. Without entering upon a resume of the various views relating to the nature of these spindlecells lying in close relation with the posterior limiting lamella, it may be stated that the most recent and trustworthy investigations, both from the morphological and the physiological standpoint, as those by Retzius and by Langley and Anderson, tend to uphold the existence of dilatator fibers if not as a continuous sheet, at least as groups of radiating fibers which collectively constitute the dilatator pupille, the presence of which as a distinct dilatator muscle may be regarded as definitely established.

The posterior limiting lamella has likewise been the subject of much discussion, due largely to the uncertain relations of the layer of delicate spindlecells occupying the iridial stroma in the immediate vicinity of the posterior pigment. The limiting lamella, or basal membrane, appears as a clear layer of reat delicacy, its maximum thickness not exceeding 0.002 mm., which closely adheres to the deeply pigmented retinal zone, with which it is often inseparably united. The lamella in question may be regarded as the attenuated anterior continuation of the membrane of Bruch, which extends forward from the choroid over the orbiculus ciliaris and ciliarv processes.

The pigment layer covering the posterior surface of the iris as far as the anterior margin of the pupil, although a conspicuous anatomical portion Of the iris, morphologically represents the anterior segment of the atrophic portion of the nervous tunic the pars retine iridica, The deeply colored layer, although ordinarily appearing as a uniform stratum of pigmentparticles, in reality consists, as seen in suitable preparations, of two distinct layers an outer, made up of low irregular fusiform elements, and an inner, composed of short polygonal cells; these lavers are continuous as the anterior margin of the pupil and represent the double layered anterior lip of the optic cup. OD approaching the ciliary processes the amount of pigment gradually lessens, first in the inner layer, and subsequently likewise in the cells of the outer layer ; filially, at the base of the ciliarv elevations the outer layer alone contains pigment particles. The inner cells are covered on their free surfaces by an extremely delicate cuticular membrane, the limitans iridis, which is probably the continuation of the cuticle investing the ciliary portion of the retinal sheet.

The blood vessels of the iris include the arterial stems given off from the anterior border of the circulus arteriosus iridis major, situated around the periphery of the iris, from which the radially disposed arterioles proceed through the stroma layer as far as the sphincter zone. At this point they freely join to form a second anastomotic circuit, the circulus arteriosus iridis nu . nor, which surrounds the pupillary opening and gives off three sets of twigs an internal, for the supply of the sphincter muscle, and anterior and posterior groups to the corresponding layers of the iris stroma.

The capillary networks derived from these sources join to form venous radicles which take a generally radial course, the veins uniting at acute angles to form the larger venous trunks which accompany those from the ciliary processes along the inner border of the ciliary muscle and terminate by joining the large venue vorticose. The vessels of the iris are provided with perivascular lymph sheaths within the thickened adventitious coat.

The lymphatics of the iris are represented by the interfascicular tissuespaces which constitute an intercom m unicati ng system of clefts within the stroma, and at the periphery communicate with the spaces within the ciliary body and with the spaces of Fontana.

The nerves of the iris are derived from the orbiculus gangliosus, which, as already noted, is formed within the accommodative muscle by the branches of the ciliary nerves. The trunks destined for the iris pursue a spiral course toward the periphery, and upon entering the stroma break lip into branches which soon become reunited, after undergoing new coin binations, to form plexuses within the stroma layer.

The nerves of the iris possess three varieties of terminal fibers : 1, motor endings ,vithin the muscular tissue; 2, sensory endings within the superficial layers of the stroma; 3, vaso motor endings within the walls of the arteries and capillaries.

The presence of ganglion nerve cells within the iris is doubtful. At best, they occur as small, sparingly distributed elements, usually of irregular multipolar form, the nervous nature of which is not beyond dispute.

The Nervous Tunic. The Retina. Viewed in the light of the more modern conceptions, the nervous coat can no longer be regarded as of the same limited morphological value as the other tunics of the eyeball, but must be considered as a true nervous center, consisting of a peripherally situated portion of the nervous system, and not merely as a complex apparatus for the perception of light stimulus.

The entire nervous tunic, as representing the structures derived from the optic vesicle, extends from the optic nerve entrance to the anterior pupillary margin. The modifications which take place within this extensive tract suffice to differentiate two sharply defined segments the posterior, embracing the hindermost part of the tunic from the optic entrance to the ora serrata, and constituting the functionating pars optica retina ; and the anterior, which includes the atrophic segment covering the posterior surface of the ciliarv body and the iris, and hence appropriately designated as the pars ciliaris and pars iridica retina respectively.

The visual portion of the nervous tunic, or retina proper, is closely applied to the choroid, and extends from the optic entrance over the posterior two thirds of the eyeball, ending abruptly at the ciliary region in a sinuous border, the ora serrata, where it passes over into the greatly attenuated anterior non visual segment of the coat.

The retina during life and in health is perfectly smooth and transparent, its blood vessels alone being distinguishable: owing to this transparency of it, inner division the dark color of the deeply pigmented outer retinal layer becomes an important factor in absorbing reflected light rays and thus preventing interference. During life the retina possesses a purplish red tint, due to the presence of the so called visual purple within certain of its elements. After death the retina soon becomes cloudy, later appearing as a thin gray veil. In thickness the retina decreases from about 0.4 mm. at the posterior pole to little over 0.2 mm. in the vicinity of the ora serrata.

On examining the eye ground (see also page 184) a conspicuous circular whitish area marks the position of the optic nerve entrance, lying a little to the nasal or inner side of the posterior pole of the eyeball. The optic disk, optic papilla, optic entrance, or porus opticus, is not quite circular, but is elliptical in form, its longest diameter being vertical and measuring about 1.7 mm. as against 1.5 mm. in the horizontal direction. The surface of the optic disk often presents a distinct funnel like depression, the physiological excavation, which results from the mode of development; the excavation is usually eccentrically placed, being somewhat toward the nasal side, where the depression is steepest and occupied by the retinal vessels. Remains of the fetal hyaloid artery may be seen as a filament of connective tissue extending into the vitreous body from the optic disk. The white appearance of the area is due to the scleral connective tissue of the lamina cribrosa and the medullated nerve fibers shining through the semi transparent layer of axis cylinders which occupy the disk.

Critically examined through the ophthalmoscope, the margin of the optic nerve appears as a faint reddish outline; next the nerve the optic disk presents a narrow white annular edge, the scleral ring, which is the edge of the aperture in the fibrous tunic; outside of the scleral border a second circle, often quite dark, and not infrequently broken, appears as an irregular pigmented zone, the choroidal ring, the presence of which is due to the deeply colored choroid. The optic entrance corresponds with the “blind spot," the explanation of which is found in the absence of the perceptive elements within this area.

The macula lutea, or yellow spot, is an area of slightly oval form distinguished by its peculiar reddish brown tint, which is due to the presence of diffused pigment particles. The macula corresponds closely with the axis of the eyeball, and lies about 4 mm. to the temporal side of the centre of the optic entrance and about 0.75 mm. below the horizontal meridian. The limits of the yellow spot are not sharply defined, since it blends into the surrounding retina, but its form, when accurately studied, is usually almost circular or but slightly elliptical, since the oval form frequently described depends, probably, more upon opbthalmoscopic appearances than upon anatomical arrangement. The greatest diameter of the yellow spot measures a little over 2 mm., and often does not quite correspond with the horizontal meridian.

About the center of the macular area a dark brown, apparently deeply pigmented spot marks the position of the fovea centralis, a depression in which the retina becomes greatly thinned, and thus allows the deeply tinted subjacent pigment to become exceptionally conspicuous. The fovea corresponds to the point of highest acuity of vision, and anatomically is distinguished by profound modifications in the arrangement of the histological elements of the retina.

The size of the fovea as usually given, between 0.2 and 0.4 mm., is too small, the recent investigations of Dimmer, Golding Bird, and Schafer indicating a diameter exceeding I mm., and, exceptionally, approximating nearly 2 mm. Owing to the absence of the rods within the fovea, and therefore, likewise, of the visual purple, this region possesses an inherently lighter tint than the surrounding retina, sometimes appearing as a faintly linted spot when examined with the ophthalmoscope. The foveal reflex seen with the mirror is due to the direction and slope of the sides of the depression, the variations in these resulting in the differences observed in the ophthalmoscopic image (compare with page 188).

The retina morphologically consists of two distinct layers an outer and inner lamella, which correspond to the external and the internal layers of the optic vesicle; the outer lamella is represented by the pigment layer, while the inner lamella includes the remaining retinal strata. The inner lamella maybe further subdivided into the neuro epithelial and the cerebral layers.

Sections of the nervous tunic, when perpendicular to the surface of the membrane, show numerous strata, the outermost of which is distinguished by its dark color, and constitutes the retinal pigment ; the succeeding lavers differ widely in the amount of protoplasmic elements which they contain, and hence vary in the intensity with which they stain, so that the retina presents lighter and darker strata when seen in usual carmine or hematoxylin preparations. The designation of the retinal layers (Fig. 33) as well as their morphological relations from without inward is as follows:

The retina may be regarded as an isolated portion of the central nervous system immediately connected with a highly specialized perceptive senseapparatus : as other parts of the nervous axis so the retina is composed of two varieties of elements, the nervous and'the sustentacular, the latter being represented by the modified neurogliar reticuluni and columns, the fibers of Miller. The nervous elements constitute collectively the ganglion retince, and represent the cortical cells of the brain. In principle, therefore, the retina consists of the percipient elements, closely applied to the pigment laver, the ganglion retine and the ganglion cells with their fibers, which establish communication with the brain centers.

The Pigment Layer. The conspicuous deeply colored stratum of pigmentcells which forms the most external laver of the retina is the direct representative of the attenuated outer lamella of the optic vesicle. It is composed of hexagonal elements, about 0.015 mm. in diameter, although subject to marked individual variation, smaller cells often surrounding larger ones. Close examination of the pigment cells in section shows that the colored particles do not invade the entire protoplasm, but that an outer zone containing the nucleus is clear, the pigment being confined to the middle and inner segments of the cells. The inner margin of the pigment cells is irregular, in contrast to the smooth external border and in close relation to the outer ends of the rod and cone segments of the visual cells (Fig. 34).

The pigment cells are profoundly affected by light stimulus, since under the influence of light the colored particles migrate toward the rods and cones, between which the protoplasm of the pigment cells extends (Fig. 35). After being subjected to darkness, on the contrary, the pigment particles are retracted and collected within the middle or so called basal zone (Fig. 36). The relation between the pigment cells and the rods and cones explains the variations in the degree of attachment between the colored and remaining portions of the retina: after exposure to light the intimate relation between the pigment and percipient elements renders the attachment between the two originally distinct lamella much stronger than that existing after seclusion in darkness, under which conditions the tendency to the natural separation of the embryologically distinct lamella becomes pronounced, the pigment then remaining attached to the choroid when the retina is removed.

The Layer of Neuro epitheliunt. Under this heading are included two strata, which are usually described as the laver of rods and cones and the external nuclear layer, the former being the specialized outer parts, and the latter the extended and attenuated nucleated bodies of the visual cells.

The layer of rods and cones represents the highly differentiated outer extremities of two forms of light perceptive elements, the rod and the cone cell. Under high amplification, as seen in section, rods of the human retina appear as elongated cylindrical forms, about 0.060 mm. in length and (1., 002 min. in thickness, each consisting of an outer and inner segment of about equal length. The outer segment possesses a uniform diameter and presents a homogeneous structure, being probably of the nature of a cuticular ge The external segments of the rods are of interest as being the chief, it' not the sole, possessor of the visual purple or rhodopsin, the color being uniformly distributed throughout this part of the rod. The inner rod segment, with slightly increased diameter, is of feebly marked, ellipsoidal form, and exhibits more or less clearly a differentiation into an external faintly striated subdivision, the rod ellipsoid, and an internal granular area, the lenticular body (Fig. 37).

The body of the rod visual cell lies within the external nuclear zone and consists of the attenuated column of protoplasm, the rod fiber, and the more conspicuous nucleus, the rod granule. The rod fiber is directly continuous with the inner part of the rod at the outer end, and extends into the external plexiform laver, within which it ends in a minute knob like expansion in close relation with the terminal arborizations of the bipolar nerve cells (Fig. 38).

The nuclei of the rod cells, which usually present transverse dark and light stripes, are of much greater thickness than the rod fiber, in consequence of which the position of the nucleus in each visual cell is indicated by a marked enlargement consisting of the nucleus surrounded by a thin envelope of protoplasm. The nuclei, or rod granules, are situated at all layers, and contribute far the larger share of' the deeply staining bodies which constitute the chief elements of the outer granule layer.

The cone visual cells are made up of the same general divisions as the associated rod elements, including the specialized outer part, the cone, and the cone cell body within the external nuclear layer. Each cone comprises an outer and an inner segment, which differ both in length and in thickness. In contrast to the almost uniform diameter and length of the two parts of the rods, the outer segment of the cones is shorter and thinner than the inner segment, which is conical, or, more accurately regarded, ellipsoidal, and measures about 0.006 mm. where it is broadest. The cones do not extend as far into the pigment layer as the rods, terminating as blunted cones at a point about opposite the middle of the outer segments of the adjacent rods. The cones do not contain the visual purple, but possess a somewhat higher refractive index than the rods. While the outer cone segment displays a tendency to break up into transverse disks, the inner segment exhibits a faint longitudinal striation.

The body of the cone visual cell contributes to form the external nuclear laver, and consists of the attenuated cell body, the cone flber and broader conspicuous nucleus, the cone granale. The latter, instead of occupying all level s of the nuclear laver, as do the nuclei of the rod cells, are limited to the zone immediately below the external limiting membrane, being continuing With the bases of the inner cone segments :' 'additional characteristics of the cone granules are their large size, lack of cross stripes, and possession of nucleoli. The cone fibers terminate within the outer plexiform lay expanded bases or feet ' which stand in close relation with the arborizations formed by the terminal expansions of the cone bipolars.

The entire number of rods within the human retina has been estimated by Krause at 130,000,000 ; that of the cones, by Salzer at 3,360,000; the number of rods, therefore in the man is greatly in excess of the cones throughout most parts of the retina in the fovea, however, the cones are alone present. The numerical proportion between the two varieties of percipient elements varies in different parts of the nervous tunic, as shown by the variation in the pattern seen on inspecting the surface of the retina where the cones appear as larger circles surrounded by areas of smaller rings the cones are usually separated by an interval occupied by three or four rods. In the vicinity of the macula the cones increase so that only a single row of rods intervene, while in the fovea the cones alone are present (Fig. 39).

The External Plexiform or Outer Reticular Layer. This stratum lies next the layer of visual cells or neuro epithelium, and is the first of the lamelle which constitute the cerebral division of the retina. The laver appears as a light, faintl y staining zone, about 0.01 mill. in breadth, the apparent granular structure of which, as seen under moderate amplification, giving place to an intricate reticulum when examined with higher magnification. The true nature of this reticulum was demonstrated only after the introduction of the more recent improved methods of staining by the Golgi silver and methylene bltie processes: recent investigations have shown that the major part of the plexiform laver consists of the delicate ramifications and intricate interlacings of the processes of the nerve cells constituting the ganglion retina and occupying the inner nuclear zone, held together by the delicate framework of sustentacular tissue.

The exact relations between the central extremities of the cone and rodvisual cells and the endings of the nerve cell processes have long been the subject of discussion. The direct connection formerly supposed to exist between the nerve cells and the visual cells is no longer tenable in the light of our modern conceptions regarding the ultimate endings of nerve processes, since the best authorities are agreed that each nerve cell exists as an independent element, whose relation to other cells is one of contiguity and not of anatomical continuity. The nervous elements in close relations with the visual cells are the " rod " and " cone " bipolars, the nucleated bodies of which form the conspicuous "granules" of the inner nuclear layer. The peripherally directed processes of these nerve cells extend within the external plexiform layer and terminate in end arborizations surrounding the inner extremities of the visual cells, which also penetrate into the reticular zone.

Additional nervous elements, the horizontal, basal, or stellate Cells, are found within the external plexiform layer; they exist in two forms, the smaller outer and the larger inner cells. The former are flattened stellate elements which lie within the outer part of the plexiform layer, through which their long axis cylinder processes extend for considerable distances to terminate in arborizations surrounding the ends of the visual cells, thus establishing indirect conduction between the elements lodged within the plexiform stratum. The larger inner horizontal cells occupy the deeper portions of the laver, some possessing descending processes which penetrate centrally as far as the inner plexiform. layer, in which they terminate in arborizations.

The Layer of Bipolar Nerve cells, or the Inner Nuclear Layer. This stratum, as usually seen, closely resembles the outer nuclear laver, being apparently composed of large numbers of deeply staining granules. The laver measures from 0.035 mm. in the vicinit of the optic disk to 0.018 mm. at the ora serrata.

The ganglion cells of the laver consist of two chief varieties those especially related to the rod visual cells, and hence appropriately called rodbipolars; and those associated with the cone cells, known as the cone bipolars. The particular purpose of the bipolars is to supply the connecting link between the visual cells, around which they terminate on the one hand, and the large ganglion elements giving off the nerve fibers to the brain, in relation to which their centrally directed processes expand, on the other. Reference to Fig. 40 shows that the arrangement of the processes of the conebipolars differs from that of the processes of the rod bipolars: the latter extend through the entire thickness of the inner plexiform layer to the bodies of the ganglion cells, which they enclose with their arborizations. The descending processes of the cone bipolars, on the contrary, are limited to the inner plexiform layer, meeting with the expansions of the ascending dendrites of the large ganglion cells at various levels, where the interlacing arborizations of the two elements form plexuses of considerable extent. The peripheral arborizations of the cone bipolars expand beneath the broad bases of the cone visual cells, forming horizontally extended, terminal plate like groups of ultimate fibrille.

In addition to the bipolar cells the inner zone of the inner nuclear layer contains nervous elements which were long ago described by Miller tinder the name of " spongioblasts," under the impression that the cells in question were concerned in the production of the sustentacular framework of the layer: these elements are now regarded as nervous in character, and, from their peculiarity of' seemingly being without axis cylinder processes, have been named by Cajal amacrine cells. The richly branching dendrites of these elements extend into the inner plexiform layer, in which they end either in the expanded brush like arborizations of the diffuse amacrines, or in the horizontally extending arborizations of the stratiform type. A few oval nuclei within this stratum belong to the long columnar supporting fibers of Muller, which usually possess irregular nucleated expansions within the zone.

The Internal Plexiform, or Inner Reticular Layer. This has been already largely described incidentally to the consideration of the bipolar cells, since the expansions of the processes of these elements contribute largely to the formation of this layer. The inner plexiform stratum, about 0.04 mm. in width, resembles closely the similar outer zone, being really an intricate reticulum formed by the interlacement of the processes of nerve cells situated in the adjacent lamina,. In addition to the delicate supporting framework of neuroglia, the principal constituents of the layer are the descending processes of the rodand cone bipolars and the horizontal cells of the inner nuclear layer, and the ascending dendrites from the subjacent large ganglion cells, augmented by the processes derived front the amacrine cells. The supporting fibers of Miller are also conspicuous as vertically coursing strie within this stratum.

The Layer of Ganglion cells. This layer, as indicated by the name, is characterized by the large nervous elements which form its chief constituent. The conspicuous ganglion cells are disposed as a closely placed single row throughout the greater part of the retina: toward the macular region, ]lowever, they become more numerous, and in the immediate vicinity of the yellow spot are arranged as a double layer, increasing in number within that area until, at the margin of the fovea, they are superimposed to such an extent that they lie from six to eight deep. Toward the ora serrata, on the contrary, they are sparingly distributed, lying isolated and widely separated. The ganglioncells resemble other typical nervous elements in the possession of richly branched dendrites, which pass into the inner plexiform layer to end in arborizations in relation with the descending processes of the bipolars, and axis cylinder processes, or neurites, which become the axis cylinders of the nerve fibers converging toward the optic entrance, and thence, as optic fibers, brainward. The details of the distribution of the dendrites within the inner plexiform layer have supplied a basis for the division of the ganglioncells into two groups those which terminate in horizontal ramifications limited to definite strata, and those which terminate in difuse ramifications distributed to the entire layer. Additional distinctions, depending on the size of the cells, as large, medium, and small, are also recognized.

The Layer of Nerve fibers. This is largely the direct contribution of the preceding stratum, since the nerve fibers composing this zone are the extended neurites of the ganglion cells. After arising from the presiding cells the fibers almost at once assume a horizontal course and form larger or smaller bundles, which, after traversing a distance varying with the position of their origin, converge to the optic entrance and contribute to the formation of the visual nerve. The size of the nerve fibers is generally small, but a limited number of very large fibers also exist: these, it is supposed, are connected with ganglion cells of exceptional magnitude.

In addition to the centrally coursing fibers the presence of fine peripherally directed, or "centrifugal," nerve fibers has been established. The central connections of such fibers are at present uncertain; their peripheral terminations lie within the inner plexiform layer, and apparently have no discoverable connection with the cells of the ganglion layer.

The bundles of nerve fibers, while pursuing a general radial course toward the optic entrance, freely intermingle and form a reticulum. The presence of the macula Iutea disturbs the strictly radial course of the bundles oil the temporal side of the optic disk, the space separating the latter from the yellow spot being traversed by from twenty five to thirty delicate fasciculi which possess an almost straight course from the macula to the disk ; these fibers collectivelv constitute the macular bundle described 1) , y Alichel. The bundles adjacent to the macular group suffer deflection from the typical radial course and arch above and below the macular area; beyond the yellow spot the arching bundles possess the typical radial arrangement.

The Sustentacular Tissue. The sus ntacular tissue, or neuroglia, of the retina exists in two forms as the conspicuous radial fibers of Muller and as the spider cells (Fig. 41).

The fibers of Muller constitute a sustaining framework which supports the nervous elements as well as the neuro epithelium, coming into intimate relations with all parts of the retina. The Mullerian fibers are modified neuroglia cells, derived originally from the ectodermal tissue of the wall of the neural tube, which extend through almost the entire thickness of the retina, reaching from the rods and cones, between which they contribute delicate septa, to the inner surface of the nervous tunic, where their expanded bases unite to form a seemingly continuous sheet, the membrana limitans interna. The fibers are slender nucleated columns which contribute lateral offshoots at various levels to the several retinal layers, among the, elements of which the processes break up into delicate sustaining fibrille and reticula. The broadest expansion along the course of the fibers usually occupies the. inner nuclear layer, and also contains the oval nucleus. At a level corresponding to the position of the inner ends of the rods and cones the sustantacular fibers come into apposition and form in apparent fenestrated partition, the membrana limitans externa, from the outer surface of which minute septa project between the rods and cones, probably acting as an insulation of the individual percipient elements. As already noted, the inner ends of the fibers of Muller are greatly enlarged, the bases of the conspicuous pyramidal or conical expansions coming into close contact and producing the appearance, when treated with silver nitrate, of a continuous layer of endothelial plates ; the bundles of retinal nervefibers pass between the diverging fibers to continue their radial course. Within the fiber layer additional sustentacular elements exist as the spidercells, neurogliar elements whose characteristic appearance is due to the long, delicate processes which extend from the cell body between the nerve fibers in various directions.

The Macula Lutea. The structure of' the retina undergoes important modifications within two areas at the macula lutea and the ora serrata (Fig. 42). On approaching the macula the ganglion cells become so numerous that a single layer no longer suffices for their accommodation, and consequently they, lie two deep ; within the macular area the number further increases, so that they constitute a stratum which includes from six to eight rows of the nervous ele¬ments. Oil reaching file fovea centralis, however, the greatly thickened gan¬glion layer rapidly decreases in thickness toward the center of the depression, becoming scattered and no longer sufficient to constitute a complete stratum, until at the bottom of the pit the ganglion cells are altogether absent. The fiber layer consequently suffers a corresponding diminution, and disappears as a distinct stratum at the point where the ganglion cells end. The bipolar cells continue to the center of the fovea as all irregular row of small elements sup¬ported within the finely reticular tissue which represents the fused outer and inner plexiform layers, and fills the space between the visual cells and the inner surface of the retina.

The most prominent stratum within the fovea is that formed by the visual cells, here composed entirely of the cone cells, which present a depth about three times that of all the more internally placed strata combined. The cones gradually lengthen oil approaching the foveal center until, over the middle of the depression, they measure more. titan double the length of the corresponding elements at the margins of the pit : associated with the increased length, the cones become greatly attenuated, appearing as long, delicate, slender fibers of which the outer segment contributes by far the greater part (Fig. 42).

The external limiting membrane exhibits a slight inward deflection over the area included within the outward curve of the inner membrane : this outer depression, the so called fovea externa produces, however, but slight dipping inward of the outer surface of the retina, as the increased length of the cones in a measure compensates for the sinking of the limiting membrane. It is probable that the position of the external fovea corresponds to an associated thickening of the choroidal tissue. In recapitulation, therefore, the layers occupying the center of the fovea are the cone visual cells ' constituting the layer of cones and the external nuclear layer and the fused outer and inner plexiform strata, with the included bipolar cells. The ganglion cells and their derivative nerve fibers are absent in the center of the fovea.

The Ora Serrata. The extreme anterior limit of the visual portion of the retina is distinguished by a sudden diminution in the thickness of the nervous tunic, dependent upon the abrupt termination of the percipient elements, as well as those layers concerned in the transmission of the light stimuli centrally, the layer of retinal pigment alone retaining its identity ill the further extension of the nervous coat.

The characteristic series of about forty well marked dentations observed in the adult retina are closely associated with the accommodative function, since in early life., before accommodation is fully exercised, the typical serrated border is wanting, the termination of the visual part of the retinal sheet being marked by a comparatively smooth line, the " transition border" of Schon, beset with numerous minute projections which afford attachment to certain of the delicate zonular fibers.

The sudden reduction of the retina depends especially upon the disappearance of the plexiform strata, the laver of rods and cones, however, having previously lost its integrity as a distinct zone. The inner nuclear layer is continued farthest, at file anterior limit of the ora passing into the single layer of columnar elements, which, in conjunction with the pigmented cells, are continued over the ciliary zone and processes as the pars ciliaris retina.

The radial fibers of Muller are especially well developed in the vicinity of the ora serrata, being of large size and numerous. So close is the relation between the sustentacular tissue and the ora that it has been suggested that the supporting fibers are continued beyond the limits of the serrated border and become connected with the zonular fibers.

The Optic Entrance. The point toward which the centrally directed axis cylinders of the fiber layer converge to escape from the interior of the eyeball and to form the optic nerve is marked by a light colored circular area, varying from 1.5 to 1.7 mm. in diameter, the optic entrance, optic disk, or optic papilla. The surface of the yellowish or bluish white disk is broken by the central retinal vessels which pierce the area eccentrically, lying usually, somewhat nearer the nasal side, and pass over the margins of the disk to gain the surrounding fiber layer.

On examining a vertical section through the optic entrance (Fig. 43) it will be seen that the thick bundles of the optic fibers which arch over the margins of the interrupted retinal and choroidal layers to gain the disk produce a slight elevation ' the papilla optic: in consequence of the rapid arching of the fibers the center of the disk is lower than the margin; hence the production of the so called physiological excavation (see also page 66). The remaining retinal layers terminate abruptly in the vicinity of the nerve entrance ' a narrow maze of reticulated intermediate tissue separating them from the arched bundles of nerve fibers. The ganglion cells are the first to disappear, while the visual cells continue farthest toward the nerve, the rod and cone fibers assuming an oblique position.

The blood vessels of the retina first appear on the optic disk as they emerge from the bundles of nerve fibers, between and parallel to which they run from the point at which they obliquely enter the optic nerve some 15 to 20 mm. beyond the eyeball. The retinal vessel 's of which the arteria centralis retina and the accompanying vein are the chief trunks, form a closed system which only indirectly, in the vicinity of the optic entrance, communicates with the vessels distributed to the remaining coats of the eyeball. On attaining the optic disk the central artery divides into two main stems, the superior and inferior pupillary branches, directed almost vertically upward and downward. These subdivide into smaller branches, the superior and inferior nasal and temporal arteries, which run mesiallv and laterally; additional twigs pass directly outward as the superior and inferior macular arteries to supply the important area of the yellow spot. While the greater part of the macular area is richly supplied with blood vessels, the fovea centralis is without them.

On examining the details of the vascular distribution of the retina it is found that the vessels of larger size are contained within the fiber laver, dividing into branches which do not anastomose, being " end arteries." The arterioles break up into rich capillary networks, which are distributed as the inner and outer plexuses, the former lying at the junction of the fiber and the ganglion layer, while the latter is situated with i n the i nner nuclear zone, bei ng especially destined for the nutrition of the functionally active bipolar nervecells. As already noted, the nutrition of the percipient elements, the visual cells, is mainly maintained by the dense vascular network of the choriocapillaris of the middle tunic.

The lymphatics of the retina are represented chiefly by the perivascular lymph channels which enclose all the veins. and capillary blood vessels, and communicate with the subpial lymph space of the optic nerve. Between the larger nerve bundles, in the vicinity of the optic papilla, the interfascicular lymph clefts may be regarded as additional lymphatic channels. The fact that injections from the subpial space pass between the pigment layer and the rods and cones, and again between the inner surface of the retina and the adjacent hyaloid membrane, has been regarded as proof of the existence of lymph spaces in these situations.

The Optic Nerve. The nerve of sight, about 5 cm. in length, is divisible into three segments the intracranial, the intraorbital, and the intraocular. The first of these, the intracranial, extends from the optic commissure to the optic foramen, a distance of about I cm., and contains the extensions of the fibers which eventually pass to end in terminal arborizations associated with the nerve cells of the cerebral centers within the pulvinar of the optic thalamus, the external geniculate bodies, and the anterior corpora quadrigemina. The cortical areas connected with sight have been definitely located within the occipital lobe, and probably include the cuneus. The intraorbital portion of the nerve presents a series of slight curves which render the nerve sigmoid rather than straight.

Transverse sections of the optic nerve show it to be composed of a large number, about eight hundred, of distinct bundles of medullated fibers separated from one another by connective tissue septa, which are derived as offshoots from the pial sheath investing the nerve. The entire number of fibers contained within the optic nerve probably approaches a million, the measurable fibers having been estimated at about half that number by Salzer. In its arrangement and composition the optic nerve resembles a gigantic funiculus, the endoneurium being in the present instance represented by the penetrating pial tissue, while the sheath itself corresponds to the perineurimn. The nerve fibers vary in diameter from a delicacy which defies measurement to a thickness of 0.01 mm. In addition to the Connective tissue fibers forming the coarser trabecula and septa, the sustentacular tissue proper consists of neuroglia in which numerous spider cells are prominent: these elements are supplemented by the deeply staining connective tissue cells belonging to the fibrous septa.

The intraorbital portion of the optic nerve is invested by extensions of the brain membranes which form the corresponding dural, arachnoidal, and pial sheaths. The general character of these envelopes is similar to that of the meninges, the tough dural sheath Iving outside and the pial sheath closely applied to the nerve, with the arachnoidal sheath between. Between the dural and arachnoidal envelopes lies the subdural lymph space; between the arachnoidal and the pial sheaths, the subarachnoidal space. On reaching the fibrous tunic of the eyeball all these sheaths, together with the included, paces, terminate by blending with the fibro elastic stroma of the sclera, the lymph spaces extending sometimes for a short distance between the fibrous bundles of the outer tunic.

The external limit of the intraocular segment of the optic nerve is distinguished by the position at which the nerve fibers acquire a medullary sheath on emerging from the sclerotic tissue which the y traverse. The scleral bundles separate to allow the passage of the groups of optic fibers, and interlace with one another to form a sieve like structure, the lamina cribrosa (Fig. 43). The bridging fibers are contributed particularly by the inner third of the scleral coat, but are supported by additional bundles of fibrous tissue derived from the connective tissue septa of the optic nerve.

The Crystalline Lens. The, most important part of the refractive apparatus of the eye consists of a transparent lenticular body, the crystalline lens, of circular outline and biconvex section, which supports the pupillary margin of the iris in front and rests within a depression, the patellar fossa, on the anterior surface of the vitreous body behind ; laterally, the lens is connected with the supporting fibers which collectively form the suspensory ligament, or zone of Zinn The lens substance consists of a soft, compressible material of such transparency during youth as to possess no color; later, with the advent of senile changes, it assumes a yellowish tint and slight opalescence, which first affects the central portion Of the lens and gradually extends toward the periphery. Early in life the lens substance is of the same consistency throughout; gradually, however, the central portion becomes harder, until in advanced age considerable difference in condensation distinguishes the Cc nucleus “from the cortical layers. The lens being non vascular, its nutrition is maintained entirely by the intercellular transmission of nutritive fluids: the differentiation of the central and peripheral portions is due to the loss of water of the favorably situated central portion of the lens. The hardening which thus gradually takes place results in loss of elasticity of the lens substance, which change is manifested in the defective accommodation which characterizes the eyes of persons after middle life. Owing to the increased density of the nucleus, the central portion of the lens of advanced years reflects more light, and the pupil consequently lacks the jet black of young eyes and appears slightly dimmed.

The soft lens substance is enclosed within a delicate elastic but strong membrane, the tens capsule: the latter is resistant to reagents, such as alcohol and acids, as well as to putrefactive changes. While possessed of considerable strength, it is brittle and readily torn by sharp instruments; when incised its cut edges roll in a characteristic manner, with the outer surface inward. When viewed in section that portion of the enveloping membrane covering the front surface of the lens is seen to be distinctly thicker than the corresponding part behind: these differences have given rise to the designation of these portions of the membrane as the anterior and pos capsule, although both are but parts of the same general envelope.

Invested by its capsule, the lens measures from 9 10 mm. in its transverse diameter, being larger in old and large subjects ; its average thickness is about 4 mm., but this dimension necessarily varies with the condition of accommodation, being somewhat greater when the eye is fixed on near objects and less when accommodated for distance. The radius of curvature of' the surfaces also varies under such changing conditions, that of the anterior stirface however, manifesting greater change under the extremes of accommodation than that of the posterior; thus, while the radii of the anterior surface for distant and near vision are respectively 10 and 6 mm., those of the posterior surface for the same conditions are respectively 6 and 5 mm. These figures establish the fact that the curvature of the anterior surface of the lens is much more affected in accommodation than that of the posterior, which remains almost unchanged. (See also page 135.) The length of a meridian of the lens measures about 12 mm. The average weight of the lens is about 0.22 gm., and the specific gravity 1121. The anterior pole of the lens lies about 2.3 min. behind the cornea under passive conditioris of accommodation; its posterior pole, about 15.5 mm. in front of the macula lutea. Critical examination has demonstrated a slight outward deviation, of from three to seven degrees, of the antero posterior lens axis from that of the eye; an additional, but smaller, vertical deviation has also been noted.

The structure of the crystalline lens can best be appreciated after recalling what has already been stated in connection with its mode of formation. The lens develops by the elongation and modification of the original ectodermic epithelial cells, which become converted into the lens fibers, those constituting the posterior wall of the primary lens sac at first composing the entire lens substance. Subsequently additional layers of lens fibers are produced by the elongation and specialization of the cells constituting the anterior wall of the lens sac, which later are known as the epithelium of the anterior capsule. The region in which the transformation of the epithelial cells into lens fibers takes place corresponds to the equatorial area, and' is known as the transitional zone; throughout the entire period of growth this region exhibits the conversion of the columnar epithelial elements of the anterior capsule into the elongated meridionally arranged lens fibers. The lens substance, therefore, is composed of modified epi¬thelial tissue.

The capsule of the lens is of entirely different origin, since its development is due to mesodermic tissues, and is distinct from that of the lens substance.

The capsule of the lens envelops the lens substance on all sides with a delicate, highly elastic membrane, which, in addition to supporting the soft material constituting the bulk of the lens, affords attachment for the fibers of the suspensory ligament. The capsule varies in thickness, being most robust in the central area of its anterior surface, where it measures from 0.010 to 0.015 mm. in thickness, and thinner at the periphery; its most attenuated part is the central area of its posterior portion, where it measures from 0.005 to 0.007 mm. The capsule does not exhibit any details of structure and in chemical composition and reactions differ from both fibrous and elastic tissue.

The epithelium of the lens capsule lies beneath the anterior capsule alone, consisting of a single layer of polyhedral flattened cells, about 0.020 mm. in diameter. These elements morphologically represent the anterior wall of the original lens sac. On approaching the margin of the lens the cells of the anterior capsule become more elongated, until finally, in the transition zone, the epithelial elements become converted into the young lens fibers. As a result of these changes being confined to a limited area, the nuclear zone, a peculiar spiral figure, is produced by the elongating cells and their nuclei, to which the term lens whorl has been applied.

The substance of the lens, constituting its entire bulk, is composed of layers of elongated and modified epitbelial cells, the lens fibers, united by an extremely thin layer of cement substance. The individual lens fibers, as seen after isolation by boiling, maceration in dilute acids, and other methods, are long, ribbon like fibers which, on transverse view, present a compressed hexagonal outline. The lens fibers vary in length, those forming the outer layers of the lens being distinctly longer than those found within the nucleus : the former extend about two thirds of the meridional distance from pole to pole, while the latter correspond to the length of the lens axis. Additional differences in the breadth and thickness exist between the fibers front the periphery and central layers, the dimensions of the more superficially situated fibers being the greater. The fibers also exhibit variations in consistency, depending upon the relatively greater amount of tissue juices in the cortical layers.

The lines of apposition of the meridionally arranged lens fibers, joined by the cement substance, produce definite figures of a stellate form, the socalled lens stars, which are especially well marked in the young or in the cortical portion of the older lens. (See page 23.)

The growth of the lens after its primary development is due entirely to the addition of lavers of new lens fibers derived exclusively from the cells of the anterior epithelium, the transformation being limited to the equatorial zone. There is no evidence of the direct multiplication of the lens fibers themselves, since these elements represent cells which have become specialized beyond the limits of reproduction.

The Vitreous Body. The extensive space bounded by the crystalline lens and its suspensory ligament in front, and by the retina behind, is filled by the vitreous body or humor vitreus. The fresh vitreous body appears as a semi fluid mass, perfectly transparent, whose general form resembles a flattened sphere, the anterior pole of which is further modified by the presence of the patellar fossa for the reception of the posterior surface of the crystalline lens. The function of the vitreous is to support the nervous tunic, rather than to act as a refractive medium, since its index of refraction (1.336) is almost identical with that of the aqueous humor, and but slightly in excess of that of water.

When the fresh vitreous is thrown upon a filter, by far the greater part of the tissue passes through as a watery fluid, a very slight proportion of the entire structure remaining as morphological Constituents : this observation establishes the fact that the vitreous body anatomically consists of' two portions, the supporting framework and the fluid tissue. Chemically, the vitreous consists of about 98.5 per cent. Water, the remaining small proportion of the whole, composed of solids, includes salts, extractives, and minute quantities of' proteids and nucleo albumin.

The semi fluid, gelatinous vitreous substance proper is enclosed within a delicate envelope, the hyaloid membrane, from which a delicate supporting reticulum extends throughout the mass of the vitreous body. Without considering in detail the conflicting views as to the structure of the vitreous body which from time to time have been advanced, it may be regarded as established that the vitreous substance represents an embryonal form of connective tissue modified by an unusual infiltration of water, so that its original condition as a connective tissue becomes masked.

The true nature of the tissue in question can only be determined by examination of the fetal vitreous before the infiltration of the watery constituents has taken place. The young tissue presents a delicate reticulation of connective tissue elements, the interlacing fibrille forming a delicate meshwork containing numerous nucleated areas. With the advance of development the connective tissue elements of the vitreous tissue become less and less conspicuous, until the adult tissue contains only suggestions of the stellate cells which at one time were prominent morphological elements. In suitably prepared specimens a delicate supporting framework composed of exceedingly fine fibrille can be demonstrated in all parts of the vitreous: at the peripheral parts of the vitreous local condensations exist which in places, as within the patellar fossa suffice to form the external limiting envelope. Membranous septa, concentrically or otherwise disposed, as described by various authors, must be regarded as artificial products if at all present.

The cellular elements of the adult vitreous (Fig. 45) are very meager, and consist in a few sparingly distributed atrophic connective tissue cells; in addition to these elements, which belong to the vitreous tissue, migratory leukocytes, or wandering cells, also occur, especially immediately beneath the hyloid membrane, where they all constitute the subgyaloid cells. These cells are derived probably from the blood vessels in the vicinity of the optic entrance and the ora serrata.

The central portion of the vitreous is penetrated by a channel, the hyloid canal, canal of Stilling, canal of Cloquet, or central canal, which extends from the optic entrance toward the lens as far as the patellar fossa : this canal surrounds the atrophic remains of the fetal hyloid vessels, which traversed the vitreous and supplied the vascular lens envelope. The channel begins as a slight enlargement, the area martegiani, of a diameter equal to that of the optic disk, and ends in the neighborhood of the posterior lens surface in a blind, not infrequently somewhat dilated, extremity.

The hyaloid membrane encloses the greater part of the vitreous body as a transparent envelope of great delicacy which closely adheres to the retina.

In eyes which have been kept for several days in dilute alcohol the hyaloid membrane can be demonstrated oil the vitreous body, since in such specimens it can be separated from the retina without mutilation. The hyaloid mem¬brane is wanting over that part of the vitreous body which surrounds the patellar fossa: within this depression the peripheral condensation of the supporting tissue of the vitreous body alone constitutes the limiting envelope of the soft gelatinous tissue within.

The Suspensory Apparatus of the Lens. The position of the crystalline lens is maintained by means of a series of delicate bands, which pass from the vicinity of the ora serrata over the ciliarv processes to be attached to the periphery of the lens. These fibers collectively constitute the suspensory ligament, or zone of Zinn, a structure of great importance not only for the support of the lens, but also in effecting the changes in the curvature of the lens surface associated with accommodation (Figs. 46 and 47).

Viewed from the posterior surface, the suspensory ligament appears as a delicate annular structure, about 6 mm. in width, which blends with the periphery of the lens on the one hand, and with the hyaloid membrane in the vicinity of the ora serrata on the other. When examined under low magnification in meridional sections of the ciliary region the suspensory ligament is seen to be not a continuous membrane, but an interlacing series of delicate fibers which bridge at various angles the space between the lens and the ciliary processes.

The older view, whereby the zone of Zinn was regarded as a direct continuation of the inner leaflet of the hyaloid membrane, formed by means of the cleavage which was supposed to take place in the vicinity of the ora serrata, has been now generally displaced by the newer teachings founded upon the more accurate studies of the developmental relations of the parts in question : according to these observations the hyaloid membrane does not undergo cleavage, but continues closely applied to the ciliary body, Over which its attenuated extension stretches as far as the processes before fading away. The suspensory fibers constituting the zone of Zinn originate as independent structures, and genetically are closely related to the primitive vitreous body. Subsequently the zonular fibers become closely attached to the ora serrata as well as the hyaloid membrane, and seemingly take partial origin from these structures (Fig. 47).

The zonular fibers of the adult may be divided into chief and accessory. The chief zonular fibers, which constitute the principal union between the lens and the surrounding ciliary body, may be subdivided into orbiculocapsular and cilio capsular according to the position of their attachment to the ciliary body, whether to the orbiculus ciliates or the ciliary processes. When traced to their attachment to the lens the fibers are found to varv in the position of their insertion into the capsule, some being fused in advance, others behind the lens periphery : these variations of attachment affect especially the orbicular group of zonular fibers, and hence their classification into the orbiculo antero capsular and the orbiculo postero eapsular fibers, which pass from the ciliary ring to the anterior and posterior surfaces of the lens capsule respectively. The fibers springing from the summits and sides of the ciliary processes join the lens capsule either on the posterior surface or at the periphery, and are hence designated the citio postero capsular or the cilio equatorial fibers.

The accessory fibers are important additions to the strength of the suspensory ligament, since they comprise numerous shorter bands which act as braces and binders to the longer chief trabecule. The accessory fibers are principally of two kinds those which pass from the ciliary processes to the long zonular fibres, and those which extend from point to point within the ciliary zone. The first group includes numerous short bands which unite the orbiculo capsular fibers with the ciliary processes and ciliary ring; the second comprises especially the bands which have the fixation of the ciliary processes as their especial purpose, and constitute two principal groups the orbiculociliary and the intraciliary fibers.

The zone of Zinn, or the suspensory ligament, is evidently not a continuous membrane, but a series of interlacing bands between which numerous apertures and clefts occur. The insertion of the zonular fibers into the lens is so regular and the fibers bound to ether so intimately that it is possible to inject air between the constituents of the zone, so that the lens is surrounded by an annular series of beaded dilatations. This appearance was long accepted as demonstrating the existence of a delicate channel) the canal of petit, encircling the periphery of the lens. With the more accurate understanding of the composition of the supporting apparatus of the lens the existence of the canal of Petit has become doubtful, and in the former sense of a closed annular channel altogether denied by most authorities. The intercommunicating spaces between the zonular fibers establish a passageway for fluids from the posterior chamber into the vitreous chamber.

The Aqueous Humor. The aqueous humor, the transparent lymph, derived from the blood vessels surrounding the spaces in which it is contained fills both the anterior and posterior chamber. as well as the extensions of the latter represented by the intrazonular spaces.

The production of the aqueous humor takes place in the posterior chamber, and is effected chiefly by the blood vessels of the ciliarv processes, and possibly also by those of the vascular ridges which extend to the posterior surface of the iris. The recesses between the ciliary processes have been regarded by some as representing special secreting tissue, the so called 11 ciliary glands," but there is little evidence to sustain the view that in the secretion of the aqueous humor the entire ciliary processes do not take part,

The quantity of aqueous humor usually present is about 275 cub. mm., its weight about 0.275 gm., and its specific gravity 1.0053. Its index of refraction is 1.337, but slightly in excess of that of water (1.334), and Dearly that of the cornea (1.360): compared with the refracting index of the viireous (1.336), it is found to be almost identical. The quantity of aqueous humor present is an important factor in determining the intraocular tension, and hence the maintenance of the free escape of the lymph, as provided for in the' spaces of Fontana and the canal of Schlenim, is of great importance. In its chemical composition the aqueous humor consists chiefly of water : in addition to the 98.6 parts of this constituent, small quantities of solids, extractives, and proteids are present. The aqueous humor possesses the property of absorbing certain organic substances with which it comes in contact, such as blood and the lens substance; it also possesses solvent properties to an extraordinary degree for many drugs. With the exception of a few migratory leukocytes, the aqueous humor is without morphological elements.

The Blood vessels of the Eyeball. The terminal arrangement and distribution of the blood vessels of the various parts of the eye have already been described in connection with the consideration of the various structures: a brief description of the general arrangement of the vessels supplying the visual organ is he added.

All the arteries supplying the eyeball are derived from the ophthalmic artery as two sets of branches, the retinal and the ciliary. These form two separate systems, which communicate only in the vicinity of the optic entrance by means of minute anastomotic twigs.

The retinal system is based upon the distribution of the central artery of the retina, a small branch which arises from the ophthalmic close to the optic foramen, usually in common with the internal ciliarv, seldom as an independent trunk. On gaining the interior of the eyeball the central stem divides into the retinal arteries, and (luring the fetal stages continues forward to the posterior lei is surface as the hyaloid artery, a vessel which later disappears.

The ciliary system supplies the remaining parts of the eyeball, and consists of two sets of vessels, the posterior and anterior ciliary arteries. The posterior arise by two chief trunks, an inner and an outer, which are given off from the ophthalmic artery while it lies below the optic nerve. These stems each divide into from four to ten branches, which surround the optic nerve, and on reaching the eyeball pierce the sclerotic coat in the vicinity of the point of entrance of the nerve. The posterior ciliary arteries then form two groups the short, which pass at once to the choroidal tract to take part in forming the rich vascular network of the middle tunic; and the long, which pass forward, one on each side of the eye, between the sclera and choroid, to the ciliarv region, where, after giving direct branches to the ciliary muscle, they join the anterior ciliary arteries to form the vascular plexuses from which the adjacent Darts are supplied.

The anterior ciliary arteries, usually from six to eight in number, are derived from the muscular and lachrymal branches of the ophthalmic; in the vicinity of the corneal margin they penetrate the scleral coat, and finally join the long posterior ciliary vessels to form the circulus arteriosus iridis major Before passing through the sclerotic these arteries give off anterior and posterior branches which supply the conjunctiva and anterior parts of the fibrous tunic. After piercing the sclera twigs are given off which pass to the ciliarv muscle, as well as others which as recurrent branches, together with similar branches from the long posterior ciliary arteries, anastomose with the choroidal vessels derived from the short ciliary trunks. An important anastomotic communication is thus established between the blood vessels supplying the choroid proper and those distributed to the ciliary region.

The branches of the long posterior and the anterior ciliary arteries inosculate within the ciliary region to form in the vicinity of the root of the iris an arterial circuit, the circulus arteriosus iridis major, from which vessels are given off to the ciliarv processes and the iris, as well as recurrent anastomotic twigs to the choroid.

The venous trunks draining the eyeball in general correspond in their arrangement to that of the arteries, the chief groups being the retinal, posterior, and anterior ciliary veins. The retinal veins receive the blood from the closed retinal system and follow closely the corresponding arteries. The posterior ciliary veins, or, more familiarly, the vene vorticose collect the blood from the iris, the ciliary processes, part of the ciliary muscle, the orbiculus ciliaris, and the choroid, and pierce the sclerotic coat within the equatorial region as four large trunks, which converge at points about equi from one another; after penetrating the fibrous tunic they additionally receive the episeleral veins. The anterior ciliarv veins drain a much more limited area than that supplied by the corresponding arteries, since within the eyeball they receive only the blood returned from the ciliary muscle, taking up the small radicles communicating with Schlemm's canal: after emerging from the sclerotic coat the anterior ciliarv veins receive as tributaries the episcleral and the anterior conjunctival vessels.

The Lymphatics of the Eyeball. The lymph cliannels of the eyehall comprise two systems, the anterior and the posterior. The anterior lymph tract embraces (1) the chambers occupied by the most important intraocular collection of lymph, the aqueous humor, together with the s stem of spaces by which this; fluid is normallv carried off, as represented by the spaces of Fontana and canal of Schlemm and (2) the elaborate system of juice channels within the cornea and adjacent part of the sclera. The posterior lymph tract includes two separate systems, that of the choroid and of the retina. The lymphatic fluid of the choroid is collected within the perichoroidal lymph space, between the choroid and the sclera, from which cleft the lymph escapes chiefly into the space of Tenon, or episcleral lymph by means of the perivascular lymphatic canals accompanying the vene vorticose : additional perivascular channels may also exist in connection with the posterior ciliary arteries. The accumulated lymph within the space of Tenon finds its way into the large intracranial lymph spaces, probably by means of the supravaginal space which surrounds the exterior of the optic nerve. The retinal system of lymphatics is represented by the perivascular lymph sheaths surrounding the' retinal vessels, as well as by the hyaloid canal within the vitreous. These channels communicate with the lymph within the optic nerve, which are connected with the great intracranial lymph space by means of the subarachnoidal spaces of the optic nerve.

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